Cross-site request forgery

Cross-site request forgery, also known as a one-click attack or session riding and abbreviated as CSRF (sometimes pronounced sea-surf[1]) or XSRF, is a type of malicious exploit of a website whereby unauthorized commands are transmitted from a user that the website trusts.[2] Unlike cross-site scripting (XSS), which exploits the trust a user has for a particular site, CSRF exploits the trust that a site has in a user's browser.

CSRF vulnerabilities have been known and in some cases exploited since 2001.[3] Because it is carried out from the user's IP address, some website logs might not have evidence of CSRF.[2] Exploits are under-reported, at least publicly, and as of 2007[4] there are few well-documented examples. The online banking webapplication of ING Direct was vulnerable for a CSRF attack that allowed illicit money transfers. [5] Popular video website YouTube was also vulnerable to CSRF in 2008 and this allowed any attacker to perform nearly all actions of any user. [5] Customers of a bank in Mexico were attacked in early 2008 with an image tag in email. The link in the image tag changed the DNS entry for the bank in their ADSL router to point to a malicious website impersonating the bank.[6]

The attack works by including a link or script in a page that accesses a site to which the user is known (or is supposed) to have been authenticated.[1] For example, one user, Alice, might be browsing a chat forum where another user, Mallory, has posted a message. Suppose that Mallory has crafted an HTML image element that references an action on Alice's bank's website (rather than an image file), e.g.,

If Alice's bank keeps her authentication information in a cookie, and if the cookie hasn't expired, then the attempt by Alice's browser to load the image will submit the withdrawal form with her cookie, thus authorizing a transaction without Alice's approval.

A cross-site request forgery is a confused deputy attack against a Web browser. The deputy in the bank example is Alice's Web browser which is confused into misusing Alice's authority at Mallory's direction.

At risk are web applications that perform actions based on input from trusted and authenticated users without requiring the user to authorize the specific action. A user who is authenticated by a cookie saved in the user's web browser could unknowingly send an HTTP request to a site that trusts the user and thereby causes an unwanted action.

CSRF attacks using image tags are often made from Internet forums, where users are allowed to post images but not JavaScript.

An attacker may forge a request to log the victim into a target website using the attacker's credentials; this is known as login CSRF. Login CSRF makes various novel attacks possible; for instance, an attacker can later log into the site with his legitimate credentials and view private information like activity history that has been saved in the account. This attack has been demonstrated against Google[7] and Yahoo.[8]

Additionally, while typically described as a static type of attack, CSRF can also be dynamically constructed as part of a payload for a cross-site scripting attack, as demonstrated by the Samy worm, or constructed on the fly from session information leaked via offsite content and sent to a target as a malicious URL. CSRF tokens could also be sent to a client by an attacker due to session fixation or other vulnerabilities, or guessed via a brute-force attack,[9] rendered on a malicious page that generates thousands of failed requests. The attack class of "Dynamic CSRF", or using a per-client payload for session-specific forgery, was described[10] in 2009 by Nathan Hamiel and Shawn Moyer at the BlackHat Briefings,[11] though the taxonomy has yet to gain wider adoption.

A new vector for composing dynamic CSRF attacks was presented by Oren Ofer at a local OWASP chapter meeting on January 2012 - "AJAX Hammer - Dynamic CSRF".[12][13]

The attacker must find a form submission at the target site, or a URL that has side effects, that does something (e.g., transfers money, or changes the victim's e-mail address or password).

The attacker must determine the right values for all the forms or URL inputs; if any of them are required to be secret authentication values or IDs that the attacker can't guess, the attack will most likely fail (unless the attacker is extremely lucky in their guess).

The attacker must lure the victim to a Web page with malicious code while the victim is logged into the target site.

Note that the attack is blind; i.e., the attacker can't see what the target website sends back to the victim in response to the forged requests, unless they exploit a cross-site scripting or other bug at the target website. Similarly, the attacker can only target any links or submit any forms that come up after the initial forged request if those subsequent links or forms are similarly predictable. (Multiple targets can be simulated by including multiple images on a page, or by using JavaScript to introduce a delay between clicks.)

Given these constraints, an attacker might have difficulty finding logged-in victims or attackable form submissions.[citation needed] On the other hand, attack attempts are easy to mount and invisible to victims, and application designers are less familiar with and prepared for CSRF attacks than they are for, say, password cracking dictionary attacks.

Synchronizer token pattern is a technique where a token, secret and unique value for each request, is embedded by the web application in all HTML forms and verified on the server side. The token may be generated by any method that ensures unpredictability and uniqueness (e.g. using a hash chain of random seed). The attacker is thus unable to place a correct token in his requests to authenticate them.[1][17][18]

STP is the most compatible as it only relies on HTML, but introduces some complexity on the server side, due to the burden associated with checking validity of the token on each single request. As the token is unique and unpredictable, it also enforces proper sequence of events (e.g. screen 1, then 2, then 3) which raises usability problem (e.g. user opens multiple tabs). It can be relaxed by using per session CSRF token instead of per request CSRF token. At the same time, it is difficult to implement web applications that make heavy use of AJAX.

JavaScript operating on the client side reads its value and copies it into a custom HTTP header sent with each transactional request

X-Csrf-Token: i8XNjC4b8KVok4uw5RftR38Wgp2BFwql

The server validates presence and integrity of the token

Security of this technique is based on the assumption that only JavaScript running within the same origin will be able to read the cookie's value. JavaScript running from a rogue file or email will not be able to read it and copy into the custom header. Even though the csrf-tokencookie will be automatically sent with the rogue request, the server will be still expecting a valid X-Csrf-Tokenheader.

The CSRF token itself should be unique and unpredictable. It may be generated randomly, or it may be derived from the session token using HMAC:

csrf_token = HMAC(session_token, application_secret)

The CSRF token cookie must not have httpOnly flag, as it is intended to be read by the JavaScript by design.

This technique is implemented by many modern frameworks, such as Django[19] and AngularJS.[20] Because the token remains constant over the whole user session, it works well with AJAX applications, but does not enforce sequence of events in the web application.

Browser extensions such as RequestPolicy (for Mozilla Firefox) can prevent CSRF by providing a default-deny policy for cross-site requests. However, this can significantly interfere with the normal operation of many websites. The CsFire extension (also for Firefox) can mitigate the impact of CSRF with less impact on normal browsing, by removing authentication information from cross-site requests. The NoScript extension for Firefox mitigates CSRF threats by distinguishing trusted from untrusted sites, and removing payloads from POST requests sent by untrusted sites to trusted ones. The Self Destructing Cookies extension for Firefox does not directly protect from CSRF, but can reduce the attack window, by deleting cookies as soon as they are no longer associated with an open tab.

Various other techniques have been used or proposed for CSRF prevention historically:

Verifying that the request's headers contain X-Requested-With (used by Ruby on Rails before v2.0 and Django before v1.2.5), or checking the HTTP Referer header and/or HTTP Origin header.[23] However, this is insecure - a combination of browser plugins and redirects can allow an attacker to provide custom HTTP headers on a request to any website, hence allowing a forged request.[24][25]

Checking the HTTP Referer header to see if the request is coming from an authorized page is commonly used for embedded network devices because it does not increase memory requirements. However, a request that omits the Referer header must be treated as unauthorized because an attacker can suppress the Referer header by issuing requests from FTP or HTTPS URLs. This strict Referer validation may cause issues with browsers or proxies that omit the Referer header for privacy reasons. Also, old versions of Flash (before 9.0.18) allow malicious Flash to generate GET or POST requests with arbitrary HTTP request headers using CRLF Injection.[26] Similar CRLF injection vulnerabilities in a client can be used to spoof the referrer of an HTTP request.

POST request method was for a while perceived as immune to trivial CSRF attacks using parameters in URL (using GET method). However, both POST and any other HTTP method can be now easily executed using XMLHttpRequest. Filtering out unexpected GET requests still prevents some particular attacks, such as cross-site attacks using malicious image URLs or link addresses and cross-site information leakage through <script> elements (JavaScript hijacking); it also prevents (non-security-related) problems with aggressive web crawlers and link prefetching.[1]

Cross-site scripting (XSS) vulnerabilities (even in other applications running on the same domain) allow attackers to bypass most CSRF preventions.[27] However, methods such as additional authentication and CAPTCHA are still effective.[28]